1. Field of the Invention
The present invention relates to a color image pickup apparatus having an image pickup element having a plurality of light-receiving elements (pixels) two-dimensionally arranged in a matrix form and, more particularly, to a color image pickup apparatus capable of outputting an image having a high resolution, reduced moire, and a high S/N ratio.
2. Related Background Art
FIGS. 1, 2, and 3 are views showing color filter arrays of conventional color solid-state image pickup elements. FIG. 1 shows a stripe filter array in which red filters R, green filters G, and blue filters B are vertically arranged to constitute the respective stripe filters. FIGS. 2 and 3 are views showing so-called mosaic filters. FIG. 2 shows a filter array in which green filters G are constituted by stripe filters, and red and blue filters R and B are vertically arranged every two filters. The set of two red filters and two blue filters is arranged parallel to the green filters G constituting each stripe filter. In a filter array of FIG. 3, magenta, green, cyan, and yellow filters Mg, Gr, Cy, and Ye are arranged in an illustrated order to constitute a unit array consisting of eight color filters, i.e., two horizontal pixels and four vertical pixels.
The image pickup elements having the above color filter arrays pose the following problems. In the image pickup element having the color filters having the array shown in FIG. 1, a color difference signal carrier is generated at a frequency 1/3 of the sampling frequency. For this reason, an image pickup operation cannot be performed up to a frequency 1/2 of the Nyquist frequency serving as the sampling frequency to decrease an image resolution.
In the image pickup element having the array shown in FIG. 2, since the R and B filters having different bands are arranged in the vertical direction, a color moire pattern tends to be formed in the vertical direction. In particular, image quality deteriorates in chromatic images.
Since the image pickup element having the color filters constituting the array shown in FIG. 3 is constituted by complementary color filters having wide bands, a color moire tends not to be formed as compared with the image pickup element having the color filters constituting the array shown in FIG. 2. However, color difference signals have low S/N ratios. When an output signal is to be quantized to perform digital processing, the color difference signals have large quantization errors, resulting in inconvenience.
In both the image pickup elements having the color filters constituting the arrays shown in FIGS. 2 and 3, color difference signal carriers are generated at a frequency 1/2 of the sampling frequency. For this reason, data up to a frequency 1/2 of the Nyquist frequency as the sampling frequency cannot be picked up.
To the contrary, there is provided an image pickup element having a color filter array called a Bayer array, as disclosed in U.S. Pat. No. 3,971,065. As shown in FIGS. 4A and 4B, assume that the horizontal pitch of pixels in the image pickup element is defined as P.sub.H and the vertical pitch of pixels in the image pickup element is defined as P.sub.V. In this case, green filters G (FIG. 4A) or luminance signal filters Y (FIG. 4B) are arranged to have an offset sampling structure having a horizontal pitch 2P.sub.H and the vertical pitch P.sub.V and offset by P.sub.H in the horizontal direction. Red and blue filters R and B are arranged to have a rectangular matrix sampling structure having a horizontal pitch 2P.sub.H and a vertical pitch 2P.sub.V. When the image pickup element having this Bayer array is used, a good image having less moire and a high S/N ratio is obtained, as is well known.
The following problem is still posed even if the image pickup elements having the Bayer arrays are used. FIGS. 5A and 5B are graphs showing the first quadrants obtained when the positions of signal carriers generated by the color filter image pickup elements shown in FIGS. 4A and 4B are plotted on the two-dimensional frequency plane (f.sub.H,f.sub.V). In the color filter image pickup element shown in FIG. 4A, an output signal from each pixel is directly switched to form a luminance signal. In the color filter image pickup element shown in FIG. 4B, only signals from pixels corresponding to the Y filters are used to form a luminance signal.
In either case, color difference signal carriers are apparently generated at (1/2P.sub.H,0) and (0,1/2P.sub.V) in the frequency space. That is, even in the image pickup element having the Bayer array, the color difference signal carrier is generated at a frequency 1/2 of the sampling frequency. Therefore, data cannot be picked up to a frequency 1/2 of the Nyquist frequency serving as the sampling frequency.
In a simple synthetic luminance signal obtained by switching output signals from the pixels or using signals from the pixels corresponding to G filters, a luminance signal having accurate spectral characteristics cannot be obtained. Therefore, the resultant luminance signal adversely affects color reproducibility or the like of an output image. For this reason, processing for substituting only a low-frequency component of the luminance signal with a luminance signal having accurate spectral characteristics has been conventionally performed. However, a circuit for forming the luminance signal having the accurate spectral characteristics becomes large in size.